feat(server): scaffolding du serveur Dart headless hexagonal (ticket #47)

Ajoute le nouveau paquet server/ (architecture hexagonale : api,
domain, application, infrastructure/postgres, infrastructure/security,
migrations, scripts) avec un premier endpoint GET /health. dart pub
get OK, dart analyze clean, dart test 1/1 vert, serveur démarré
manuellement et validé (GET /health -> 200 {"status":"ok"}).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

server/bin/server.dart

@@ -0,0 +1,17 @@
+import 'dart:io';
+
+import 'package:gametime_server/api/router.dart';
+import 'package:shelf/shelf_io.dart' as shelf_io;
+
+Future<void> main(List<String> arguments) async {
+  final port = int.tryParse(Platform.environment['PORT'] ?? '') ?? 8080;
+  final server = await shelf_io.serve(
+    buildApiHandler(),
+    InternetAddress.anyIPv4,
+    port,
+  );
+
+  print(
+    'GameTime server listening on ${server.address.address}:${server.port}',
+  );
+}